1. Field of the Invention
The present invention relates to synchronous digital transmission at very high data rates using cables and repeaters.
2. Description of the Prior Art
This type of transmission usually employs a line signal with no DC or low-frequency components, these usually not being transmitted by the repeaters and reserved for remote power feed and remote supervisory purposes; the signal is made up a synchronous sequence of multivalent symbols following on from one another at the so-called modulation rate and meeting the criteria established by Nyquist for transmission without intersymbol interference. The synchronous sequence of multivalent symbols is obtained by employing low-pass filtering in accordance with the Nyquist criteria to shape the synchronous sequence of their levels which is a signal with no DC or low-frequency component resulting from a so-called line coding operation. The maximum length that may be assigned to the regeneration step, that is to say the distance that the symbols travel over the cable beyond which they can no longer be reliably recognized, depends on the attenuation per unit length of the cable which is in turn dependent on the modulation rate.
In attempting to increase the length of the regeneration step keeping the same digital data rate or to increase the digital data rate keeping the same regeneration step it is necessary to combat the attenuation due to the cable, to which problem there are three solutions: to increase the transmit level, that is to say the amplitude of the symbols at the output from each repeater, or to reduce the modulation rate by increasing the number of symbol levels, or to tolerate a controled amount of intersymbol interference.
The facility for increasing the transmit level is in fact somewhat limited because any increase in the transmit level is accompanied by an increase in the power dissipation in the repeaters and in non-linear phenomena causing crosstalk.
Increasing the number of symbol levels by the use of multilevel line coding degrades the signal-to-noise ratio on which the error rate is dependent and, more importantly, increases the complexity of the repeaters, which depends on the number of levels to be regenerated, as well as causing operating difficulties because of the sensitivity of these codes, increasing in proportion to the number of levels, to jitter and to variations in attenuation and phase due to the cable. In practise the number of levels is limited to three, ternary line codes being by far and away those most commonly used on high data rate digital transmission systems using cables and repeaters, and an attempt is merely made to reduce the modulation rate by reducing the redundancy of the binary-ternary line codes used while preserving minimal timing information, absence of DC and low-frequency components and an error checking facility. In this line of thinking it would seem that there is no possibility of development beyond the 6B/4T line code which, in the digital link described in the journal Commutation et Transmission No 2, 1984, pages 47 through 66, makes it possible to achieve a regeneration step of 4.5 km on a 2.6/9.5 mm coaxial cable with a data rate of 140 Mbit/s.
Tolerance of a controled degree of intersymbol interference is a characteristic of the partial response coding technique described in U.S. Pat. No. 3,388,330 in particular.
This technique, used to increase the data rate on a link, consists in modelling the passband of the transmission channel in such a way that the latter responds only partially during the duration of a symbol and features some degree of memory effect leading to linear superposition of symbols that can be decrypted. Response coding leads, like multilevel line coding, to degradation of the signal-to-noise ratio and entails decrypting of linear superposition of symbols at each repeater, but it is less sensitive to jitter and to phase variations due to the cable and does not increase the number of levels of the signal to be regenerated.
The partial response code is not used as such, as it is believed to increase the error rate, the decrypting of the linear superposition of symbols received relying on the values of symbols previously received. It is systematically associated with precoding as described in the aforementioned U.S. Pat. No. 3,388,330 and in U.S. Pat. No. 3,492,578, which makes it possible to decrypt linear superposition of symbols without any knowledge of the symbols received previously by making the level resulting from linear superposition correspond, on the transmit side, to the level of the symbols to transmit modulo N, where N is the number of possible symbol levels.
Precoding is a non-linear process, however, and has the disadvantage that it modifies the frequency spectrum of the sequence of symbols transmitted and makes it necessary to attempt suppression of DC and low-frequency components by the partial response coding process itself. This imposes the use of a class 4 partial response system which is not the most advantageous from the point of view of sensitivity to jitter and to variations in attenuation and due to the cable phase.
An objective of the present invention is to provide a synchronous digital cable transmission system using repeaters having a longer regeneration step by virtue of a combination of the multilevel line coding and partial response coding techniques.